Many hydrocarbon gases such as natural gas, cracked gas, or refinery off gas contain one or more light components that either contaminate the main gas or that are themselves valuable if they can be separated from the main gas stream. Such light gases include nitrogen, helium, and hydrogen. A number of economic considerations make it desirable to separate these light gases from a hydrocarbon gas stream.
For example, contamination of natural gas with one or more light components is particularly common. Natural gas is a mixture of hydrocarbons, including methane ethane, propane, butane and pentane. Natural gas can also contain nitrogen, helium, and acid gases such as carbon dioxide and hydrogen sulfide. Nitrogen is sometimes a natural component or may derive from nitrogen injections utilized for reviving oil wells in suitable formations. Helium occurs naturally in a small portion of natural gas reservoirs. Natural gas must meet certain criteria for acid gas content, heating value, dew point, and total inert content before the natural gas can be transported and marketed. Nitrogen content is often limited to less than 2-4% molar. Nitrogen must therefore be removed from natural gas containing more than the specified amount or the natural gas cannot be transported and marketed.
Natural gas is also produced in association with crude oil production as associated gas. This associated gas may contain naturally occurring nitrogen or may contain injected nitrogen used to enhance oil recovery. Associated gas must meet the same criteria as natural gas if the associated gas is to be transported and marketed.
Refinery and chemical plant streams often contain a number of light components such as nitrogen and hydrogen. Hydrogen is commonly contained in gas streams in refinery units. Hydrogen is added to some refinery operations and is produced as a side-product in other refinery unit operations. It is often desirable to separate this hydrogen from the refinery off gas because removed and recovered hydrogen can be recycled within the facility or sold, typically for a higher value than the heating value of the hydrogen in a refinery or chemical plant hydrocarbon stream. Likewise, removing nitrogen from the plant stream increases the heating value of the remaining hydrocarbon stream and potentially increases the stream's value as a fuel stream.
Separation of light components such as hydrogen or nitrogen from heavier components such as methane and ethane can increase the value of either or both of the resulting separate streams. Existing technologies for performing such separations include the use of selective membranes, adsorption systems such a pressure swing adsorption, and systems that utilize very low temperatures (cryogenic plants) such as expander, Joule-Thompson, or cascaded refrigeration plants.
Absorption using a physical solvent to remove the heavier components and therefore separate them from the light components, a process known as the Mehra Process™, can be employed. The Mehra Process is described in several U.S. patents, including U.S. Pat. Nos. 4,623,371, 4,832,718, 4,833,514, and 5,551,972, which are hereby incorporated herein by reference. These patents describe systems for absorption/flash regeneration systems for removal of light components such as nitrogen or hydrogen from heavier components such as methane or ethylene. They address systems wherein the physical solvent used is external, meaning a made up of component(s) added to the system, and also systems wherein the physical solvent used is internally generated and is therefore composed of heavier component(s) in the feed gas. An improvement to these processes is also described in U.S. Pat. No. 6,698,237B2 by Thomas K. Gaskin, which addresses use of stripping gas to enhance the performance of flash regeneration systems. A further improvement is described in U.S. patent application Ser. No. 11/076,356 (incorporated herein by reference) by Thomas K. Gaskin, which describes the use of cryogenic temperatures in processing gases in solvent absorption systems.
In the processes described in the above paragraphs, the heavier components are absorbed away from the light component(s) using a circulating physical solvent. Reducing the pressure of the rich solvent in a flash separator releases the heavier component and regenerates the solvent for recirculation to the absorber. The physical solvent may be a liquid chosen for its physical properties, one property being that it is heavier than the component to be absorbed from the light component. The physical solvent may also be made up entirely of the heaviest components of the feed gas stream. These heaviest components are those that do not readily vaporize in the flash regeneration of the circulating solvent. These absorption processes are characterized in that a feed stream comprising multiple components enters the process and two or more streams, each being enriched in at least one of the components, leaves the process. Any improvement to the process that results in, 1) increased process efficiency, or 2) reduced process implementation cost, or 3) improved operability, or 4) increased the purity of one or more of the exiting streams will be appreciated as a technical contribution to the art.